I/O Collections random access

This pull request implements xp-framework/rfc#196 and adds the following new methods to `io.collections.FileCollection`:

```groovy
  public io.collections.IOElement newElement(string $name)
  public io.collections.IOCollection newCollection(string $name)
  public io.collections.IOElement findElement(string $name)
  public io.collections.IOCollection findCollection(string $name)
  public io.collections.IOElement getElement(string $name) throws util.NoSuchElementException
  public io.collections.IOCollection getCollection(string $name) throws util.NoSuchElementException
  public void removeElement(string $name)
  public void removeCollection(string $name)
```

All methods may also throw `IOException`s.